Sean Smith has an interesting article up at The Hardball Times about replacement level. The whole thing is worth reading, but I want to highlight one cool table:
-22.5 2B, 3B, CF
-12.5 Corner Outfield
-7.5 First base
The number listed for each position is how many runs below average a player must be offensively to be considered replacement level (assuming average fielding ability).
So, at -7.5, first basemen have to hit just worse than major league average just to provide any value. On the other hand, catchers can be more than 30 runs worse than average and still deserve a spot in the majors. That's a 25 run difference between the two positions, which should explain why first basemen are extremely overrated.
* Added by Sky, not in Sean's article.